1. Field of the Invention
This invention relates to novel compounds which are thyroid receptor ligands, and are preferably selective for the thyroid hormone receptor xcex2, to methods of preparing such compounds and to methods for using such compounds such as in the regulation of metabolism.
2. Brief Description of the Prior Art
While the extensive role of thyroid hormones in regulating metabolism in humans is well recognized, the discovery and development of new specific drugs for improving the treatment of hyperthyroidism and hypothyroidism has been slow. This has also limited the development of thyroid hormone agonists and antagonists for treatment of other important clinical indications, such as hypercholesterolemia, obesity and cardiac arrhythmias.
Thyroid hormones affect the metabolism of virtually every cell of the body. At normal levels, these hormones maintain body weight, the metabolic rate, body temperature, and mood, and influence serum low density lipoprotein (LDL) levels. Thus, in hypothyroidism there are weight gain, high levels of LDL cholesterol, and depression. In excess with hyperthyroidism, these hormones lead to weight loss, hypermetabolism, lowering of serum LDL levels, cardiac arrhythmias, heart failure, muscle weakness, bone loss in postmenopausal women, and anxiety.
Thyroid hormones are currently used primarily as replacement therapy for patients with hypothyroidism. Therapy with L-thyroxine returns metabolic functions to normal and can easily be monitored with routine serum measurements of levels of thyroid-stimulating hormone (TSH), thyroxine (3,5,3xe2x80x2,5xe2x80x2-tetraiodo-L-thyronine, or T4) and triiodothyronine (3,5,3xe2x80x2-triiodo-L-thyronine, or T3). However, the rapidity with which replacement therapy can be given and in some circumstances, particularly in older individuals, even replacement therapy, is limited by certain of the deleterious effects of thyroid hormones.
In addition, some effects of thyroid hormones may be therapeutically useful in non-thyroid disorders if adverse effects can be minimized or eliminated. These potentially useful influences include weight reduction, lowering of serum LDL levels, amelioration of depression and stimulation of bone formation. Prior attempts to utilize thyroid hormones pharmacologically to treat these disorders have been limited by manifestations of hyperthyroidism, and in particular by cardiovascular toxicity.
Development of specific and selective thyroid hormone receptor agonists could lead to specific therapies for these common disorders while avoiding the cardiovascular and other toxicities of native thyroid hormones. Tissue-selective thyroid hormone agonist may be obtained by selective tissue uptake or extrusion, topical or local delivery, targeting to cells through other ligands attached to the agonist and targeting receptor subtypes. Thyroid hormone receptor agonists that interact selectively with the xcex2-form of the thyroid hormone receptor offers an especially attractive method for avoiding cardio-toxicity.
Thyroid hormone receptors (TRs) are, like other nuclear receptors, single polypeptide chains. The various receptor forms appear to be products of two different genes xcex1 and xcex2. Further isoform differences are due to the fact that differential RNA processing results in at least two isoforms from each gene. The TRxcex11, TRxcex21 and TRxcex22 isoforms bind thyroid hormone and act as ligand-regulated transcription factors. In adults, the TRxcex21 isoform is the most prevalent form in most tissues, especially in the liver and muscle. The TRxcex12 isoform is prevalent in the pituitary and other parts of the central nervous system, does not bind thyroid hormones, and acts in many contexts as a transcriptional repressor. The TRxcex11 isoform is also widely distributed, although its levels are generally lower than those of the TRxcex21 isoform. This isoform may be especially important for development. Whereas many mutations in the TRxcex2 gene have been found and lead to the syndrome of generalized resistance to thyroid hormone, mutations leading to impaired TRxcex1 function have not been found.
A growing body of data suggest that many or most effects of thyroid hormones on the heart, and in particular on the heart rate and rhythm, are mediated through the xcex1-form of the TRxcex11 isoform, whereas most actions of the hormone such as on the liver, muscle and other tissues are mediated more through the xcex2-forms of the receptor. Thus, a TRxcex2-selective agonist might not elicit the rhythm and rate influences of the hormones but would elicit many other actions of the hormones. It is believed that the xcex1-form of the receptor is the major drive to heart rate for the following reasons:
1) tachycardia is very common in the syndrome of generalized resistance to thyroid hormone in which there are defective TRxcex2-forms, and high circulating levels of T4 and T3;
2) there was a tachycardia in the only described patient with a double deletion of the TRxcex2 gene (Takeda et al, J. Clin. Endrocrinol. and Metab. 1992, Vol. 74, p. 49);
3) a double knockout TRxcex1 gene (but not xcex2-gene) in the mouse has a slower pulse than control mice; and,
4) western blot analysis of human myocardial TR""s show presence of the TRxcex11, TRxcex12 and TRxcex22 proteins, but not TRxcex21.
If these indications are correct, then a TRxcex2-selective agonist could be used to mimic a number of thyroid hormone actions, while having a lesser effect on the heart. Such a compound may be used for: (1) replacement therapy in elderly subjects with hypothyroidism who are at risk for cardiovascular complications; (2) replacement therapy in elderly subjects with subclinical hypothyroidism who are at risk for cardiovascular complications; (3) obesity; (4) hypercholesterolemia due to elevations of plasma LDL levels; (5) depression; and, (6) osteoporosis in combination with a bone resorption inhibitor.
In accordance with the present invention, compounds are provided which are thyroid receptor ligands, and have the general formula I: 
in which:
R1 is alkyl of 1 to 6 carbons or cycloalkyl of 3 to 7 carbons;
R2 and R3 are the same or different and are hydrogen, halogen, alkyl of 1 to 4 carbons or cycloalkyl of 3 to 5 carbons, at least one of R2 and R3 being other than hydrogen;
n is an integer from 0 to 4;
R4 is an aliphatic hydrocarbon, an aromatic hydrocarbon, carboxylic acid or ester thereof, alkenyl carboxylic acid or ester thereof, hydroxy, halogen, cyano, or a phosphonic acid or ester thereof, or a pharmaceutically acceptable salt thereof, and all stereoisomers thereof.
In addition, in accordance with the present invention, a method for preventing, inhibiting or treating a disease associated with metabolism dysfunction or which is dependent upon the expression of a T3 regulated gene is provided, wherein a compound of formula I is administered in a therapeutically effective amount. The compound of formula I is preferably an agonist that is preferably selective for the thyroid hormone receptor-beta. Examples of such diseases associated with metabolism dysfunction or are dependent upon the expression of a T3 regulated gene are set out hereinafter and include obesity, hypercholesterolemia, atherosclerosis, cardiac arrhythmias, depression, osteoporosis, hypothyroidism, goiter, thyroid cancer as well as glaucoma and congestive heart failure.
The following definitions apply to the terms as used throughout this specification, unless otherwise limited in specific instances.
The term xe2x80x9cthyroid receptor ligandxe2x80x9d as used herein is intended to cover any moiety which binds to a thyroid receptor. The ligand may act as an agonist, an antagonist, a partial agonist or a partial antagonist.
The term xe2x80x9caliphatic hydrocarbon(s) as used herein refers to acyclic straight or branched chain groups which include alkyl, alkenyl or alkynyl groups.
The term xe2x80x9caromatic hydrocarbon(s) as used herein refers to groups including aryl groups as defined herein.
Unless otherwise indicated, the term xe2x80x9clower alkylxe2x80x9d, xe2x80x9calkylxe2x80x9d or xe2x80x9calkxe2x80x9d as employed herein alone or as part of another group includes both straight and branched chain hydrocarbons, containing 1 to 12 cartons (in the case of alkyl or alk), in the normal chain, preperably 1 to 4 carbons, such as methyl, ethyl, propyl, isopropyl, butyl, t-butyl, or isobutyl, pentyl, hexyl, isohexyl, heptyl, 4,4-dimethylpentyl, octyl, 2,2,4-trimethylpentyl, nonyl, decyl, undecyl, dodecyl.
The term xe2x80x9carylxe2x80x9d as employed herein alone or as part of another group refers to monocyclic and bicyclic aromatic groups containing 6 to 10 carbons in the ring portion (such as phenyl or naphthyl including 1-naphthyl and 2-naphthyl) and may be optionally substituted through available carbon atoms with 1, 2, or 3 groups selected from hydrogen, halo, alkyl, haloalkyl, alkoxy, haloalkoxy, alkenyl, trifluoromethyl, trifluoromethoxy, alkynyl, hydroxy, nitro or cyano.
Unless otherwise indicated, the term xe2x80x9clower alkenylxe2x80x9d or xe2x80x9calkenylxe2x80x9d as used herein by itself or as part of another group refers to straight or branched chain radicals of 2 to 12 carbons, preferably 2 to 5 carbons, in the normal chain, which include one to six double bonds in the normal chain, such as vinyl, 2-propenyl, 3-butenyl, 2-butenyl, 4-pentenyl, 3-pentenyl, 2-hexenyl, 3-hexenyl, 2-heptenyl, 3-heptenyl, 4-heptenyl, 3-octenyl, 3-nonenyl, 4-decenyl, 3-undecenyl, 4-dodecenyl, and the like.
Unless otherwise indicated, the term xe2x80x9clower alkynylxe2x80x9d or xe2x80x9calkynylxe2x80x9d as used herein by itself or as part of another group refers to straight or branched chain radicals of 2 to 12 carbons, preferably 2 to 8 carbons, in the normal chain, which include one triple bond in the normal chain, such as 2-propynyl, 3-butynyl, 2-butynyl, 4-pentynyl, 3-pentynyl, 2-hexynyl, 3-hexynyl, 2-heptynyl, 3-heptynyl, 4-heptynyl, 3-octynyl, 3-nonynyl, 4-decynyl,3-undecynyl, 4-dodecynyl and the like.
Unless otherwise indicated, the term xe2x80x9ccycloalkylxe2x80x9d as employed herein alone or as part of another group includes saturated cyclic hydrocarbon groups or partially unsaturated (containing 1 or 2 double bonds) cyclic hydrocarbon groups, containing one ring and a total of 3 to 7 carbons, preferably 3 to 5 carbons, forming the ring, which includes cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl, cyclopentenyl and cyclohexenyl.
The term xe2x80x9chalogenxe2x80x9d or xe2x80x9chaloxe2x80x9d as used herein alone or as part of another group refers to chlorine, bromine, fluorine, and iodine as well as CF3, with chlorine or bromine being preferred.
The term xe2x80x9cphosphonic acidxe2x80x9d refers to a phosphorus containing a group of the structure 
wherein R5 is H or lower alkyl.
The compounds of formula I can be present as salts, in particular pharmaceutically acceptable salts. If the compounds of formula I have, for example, at least one basic center, they can form acid addition salts. These are formed, for example, with strong inorganic acids, such as mineral acids, for example sulfuric acid, phosphoric acid or a hydrohalic acid, with strong organic carboxylic acids, such as alkanecarboxylic acids of 1 to 4 carbon atoms which are unsubstituted or substituted, for example, by halogen, for example acetic acid, such as saturated or unsaturated dicarboxylic acids, for example oxalic, malonic, succinic, maleic, fumaric, phthalic or terephthalic acid, such as hydroxycarboxylic acids, for example ascorbic, glycolic, lactic, malic, tartaric or citric acid, such as amino acids, (for example aspartic or glutamic acid or lysine or arginine), or benzoic acid, or with organic sulfonic acids, such as (C1-C4)-alkyl- or aryl-sulfonic acids which are unsubstituted or substituted, for example by halogen, for example methyl- or p-toluene-sulfonic acid. Corresponding acid addition salts can also be formed having, if desired, an additionally present basic center. The compounds of formula I having at least one acid group (for example COOH) can also form salts with bases. Suitable salts with bases are, for example, metal salts, sucn as alkali metal or alkaline earth metal salts, for example sodium, potassium or magnesium salts, or salts with ammonia or an organic amine, such as morpholine, thiomorpholine, piperidine, pyrrolidine, a mono-, di- or tri-lower alkylamine, for example ethyl-, tert-butyl-, diethyl-, diisopropyl-, triethyl-, tributyl- or dimethyl-propylamine, or a mono-, di- or trihydroxy lower alkylamine, for example mono-, di- or triethanolamine. Corresponding internal salts may furthermore be formed. Salts which are unsuitable for pharmaceutical uses but which can be employed, for example, for the isolation or purification of free compounds I or their pharmaceutically acceptable salts, are also included.
Preferred salts of the compounds of formula I which include a basic gorup include monohydrochloride, hydrogensulfate, methanesulfonate, phosphate or nitrate.
Preferred salts of the compounds of formula I which include an acid group include sodium, potassium and magnesium salts and pharmaceutically acceptable organic amines.
Preferred are compounds of the invention of formula I wherein R1 is isopropyl;
R2 and R3 are independently halogen such as bromo or chloro; or
R2 and R3 are each methyl or one is methyl and the other is ethyl;
or one of R2 and R3 is halogen such as bromo or chloro, and the other is alkyl such as methyl, or hydrogen;
n is 0, 1 or 2; and
R4 is carboxylic acid (COOH) or esters thereof, alkenyl carboxylic acid or esters thereof, OH, CN, halogen such as iodo, phosphonic acids or esters thereof such as 
Preferred compounds of the invention have the structures 
The compounds of formula I may be prepared by the exemplary processes described in the following reaction schemes. Exemplary reagents and procedures for these reactions appear hereinafter and in the working Examples.
Compounds of formula I of the invention can be prepared using the sequence of steps outlined in Schemes 1 to 5 set out below.
In Scheme 1, an anisole-derived iodonium salt 2 and copper bronze in an inert solvent such as dichloromethane are mixed at room temperature. A mixture of the appropriate phenol ester 1 and a base such as triethylamine in an inert solvent such as dichloromethane was added to the mixture, generally using 2 molar equivalents each of the phenol and base, and 3 molar equivalents of iodonium salt 2. After stirring overnight at room temperature, the reacted mixture is purified via chromatography on silica gel, to give biaryl ether products 3. Other methods exist in the literature for the synthesis of diaryl ethers, for example, two references directly apply to the synthesis of thyroid hormone analogs: D. A. Evans et al., Tet. Letters, volume 39, 2937-2940 (1998) and G. M. Salamonczyk et al., Tet. Letters, volume 38, 6965-6968 (1997). The carboxylic acid ester is removed with a mixture of aqueous sodium hydroxide and methanol. Acidification of the completed reaction mixture is followed by standard work-up and crystallization or chromatography. The methyl ether function is removed by treatment of the free acid product of the previous procedure with 4-6 molar equivalents of a strong acid such as boron tribromide at 0xc2x0 C. in an inert solvent such as dichloromethane. The reacted mixture gives after standard work-up and purification, the end product 4 (Examples 1, 3, 4, 5 and 11). Other combinations of protecting groups for the carboxylic acid present in 1 and phenolic hydroxyl in iodonium salt 2 can be employed, and their usage is known to those skilled in the art (references describing protecting group strategy include, for example, xe2x80x9cProtecting Groups in Organic Chemistryxe2x80x9d, J. F. W. McOmie, Plenum Press, London, New York, 1973, and xe2x80x9cProtective Groups in Organic Synthesisxe2x80x9d, T. W. Greene, Wiley, New York, 1984).
Examples of compounds of formula I in which R4=OH can be prepared by further chemistry are depicted in Scheme 1. The intermediate ester product 3 can reduced by treatment with an appropriate reducing agent such as diisobutyl aluminium hydride in an inert solvent such as THF at 0xc2x0 C. If R2 and R3 are alkyl, then lithium aluminum hydride may be employed without the risk of reducing away halogen substituents at those positions. Standard work-up and purification yields the desired alcohol product 5. Other reducing agents may be employed and are known to those skilled in the art. Removal of the phenolic protecting group as described above affords the final product alcohol 6, which are compounds of formula I in which R4=OH (Example 6).
Intermediate 5 in Scheme 1 may be converted to compounds of formula I in which R4=halogen by any one of a number of sequences well known to those skilled in the art. For example, 2 molar quivalents of sodium iodide can be added to a mixture of alcohol 5, P2O5 and H3PO4, and heated at 120xc2x0 C. for 15 minutes, to give the intermediate iodide 7. Removal of the phenolic protecting group gives final product 8, a compound of formula I in which R4=iodide (Examples 2 and 7) after conventional work-up and purification. Numerous other methodologies for conversion of simple hydroxyl groups to the corresponding alkyl halide are well known to those skilled in the art.
Various extended carboxylic acid compounds of formula I (R4=COOH) can be obtained from intermediate 7 in Scheme 1. For example, the anion of ethyl malonate can be stirred with alkyl iodide 7 overnight at reflux in a polar solvent such as t-butanol or dimethylformamide, employing a molar ratio of the anion, compared with the iodide, within the range of 2-3 to 1. After hydrolysis of the alkylated diester product of this reaction, the corresponding monoacid is obtained by heating the diacid to temperatures around 180xc2x0 C. Removal of the methyl ether protecting group as described above yields the desired product 9 in which R4=COOH (Example 8).
Compounds of formula I in which R4=CN can be obtained by reacting intermediate 7 in Scheme 1 with sodium cyanide in a polar solvent or solvent mixture such as water:ethanol (1:3). The reaction may be stirred at reflux and the amount of sodium cyanide employed may be at least 5 times molar excess relative to the iodide 7 in order to drive the reaction to completion. Work-up and purification of the resulting product by standard means affords the desired nitrile product. Removal of the methyl ether group by methods described above yields the final product 10 in which R4=CN (Example 9). The nitrile final products may also be converted to the corresponding carboxylic acids by standard hydrolytic procedures such as heating to around 100xc2x0 C. in a mixture comprised of equal volumes of acetic acid, sulfuric acid and water. Standard work-up and purification affords the corresponding carboxylic acid product (Example 10). 
The procedures described in Scheme 2 further exemplify methods for the synthesis of compounds of formula I. For example, the unprotected phenol, carboxylic acid intermediate 10 in Scheme 2 may be reduced by standard means known in the art, such as reacting with 3 molar equivalents of tetrabutylammonium boronate and 4 molar equivalents of ethylbromide in a solvent mixture consisting of dichloromethane:THF:water (5:2:2). Work-up and purification by standard means affords product 11 which are compounds of the formula I in which R4=OH (Examples 12 and 16). The alcohol products 11 can be reacted with 0.5 molar equivalents PBr3 in an inert solvent such as dichloromethane at room temperature. The reacted mixture after work-up and purification yields the corresponding bromides 12 (Examples 13a and 17a). These alkyl bromides may be converted to compounds of formula I in which R4=CN by reaction with sodium cyanide in a polar solvent or solvent mixture such as water:DMF (1:9) and heating at 50xc2x0 C. The reacted mixture gives after work-up and chromatography the benzylcyanide products 13 (Example 13 and 17). In addition, the bromide intermediates 13 can be converted to compounds of formula I in which R4=phosphonic acid diesters by reaction with trialkylphosphites under standard Arbusov reaction conditions. For example, the reaction of Example 13a with triethyl phosphite in large excess (10-12 molar equivalents versus bromide 12) in toluene at reflux for at least 2 days yields the corresponding dialkylphosphonate ester, compound 14 (Example 14) in Scheme 2. Simple hydrolysis of this product in aqueous acid solvent such as in hot hydrochloric acid gives the corresponding phosphonic acid 15 (Example 15) after work-up and purification. 
Scheme 3 depicts a synthesis of compounds of formula I in which R4=COOH and is connected to the aromatic ring by an intervening double bond (alkenyl carboxylic acid). A bromophenol of general structure 16 is coupled to iodonium salt 17 in the same manner as described above to give the diaryl ether product 18. Reaction of diaryl ether 18 with an acrylate ester such as ethyl acrylate, using palladium acetate, triphenyl phosphine and triethylamine in a solvent such as acetonitrile with heating at elevated temperatures gives a cinnamate ester product which, after removal of the ester and methyl ether as described previously, gives the product alkenyl carboxylic acid 19 (Example 18). 
Examples of compounds of formula I in which R2 is different from R3 can be synthesized by the method shown in Scheme 4. Standard halogenation conditions are well-known to those versed in the art to convert the tri-substituted phenol ester intermediate 20 to product 21 in which R3 is halogen and is different from R2. For example, iodination can be achieved by using iodine in glacial acetic acid in the dark. Similarly, bromination can be accomplished by substituting bromine under the same reaction conditions. The product phenol 21 is then coupled to iodonium salt 2 to give the diaryl ether product 22. The ester group R and the methyl ether protecting groups are removed by procedures already described above to give the final product 23 where R3 is halogen and is different from R2 (Examples 19, 20 and 26). 
Compounds of formula I in which R2=Cl and R3=alkyl can be prepared by the methods shown in Scheme 5. The intermediate 24 in which R2=Cl and R3 may be either Br of I can be alkylated under standard Stille conditions such as using tetramethyl tin [(Me)4Sn] or tetraethyl tin [(Et)4Sn] in the presence of tetrakis(triphenylphosphine)palladium in a solvent such as toluene. Heating this mixture in the dark under an inert atmosphere yields, after normal work-up and purification, the product 25 in which R2=Cl and R3=alkyl. Removal of the methyl ether and ester protecting groups under standard conditions well-known to those versed in the art yields the final product 26 (Examples 21 and 22).
The reduction product 27 (Example 23) was obtained under the reaction conditons described in Scheme 5. 
With respect to the above reaction schemes, although the various R1, R2, R3, R4 and n moieties are specifically defined, unless otherwise indicated, it is to be understood that R1, R2, R3, and R4 may be any of the groups encompassed thereby and n may be 0, 1, 2, 3 or 4.
The compounds of the invention are agonists, that are preferably selective for the thyroid hormone receptor-beta, and as such are useful in the treatment of obesity, hypercholesterolemia and atherosclerosis by lowering of serum LDL levels, alone or in combination with a cholesterol lowering drug such as an HMG CoA reductase inhibitor, amelioration of depression alone or in combination with an antidepressant, and stimulation of bone formation to treat osteoporosis in combination with any known bone resorption inhibitor such as alendronate sodium. In addition, the compounds of the invention may be useful as replacement therapy in elderly patients with hypothyroidism or subclinical hypothyroidism who are at risk for cardiovascular complications, and in the treatment of non-toxic goiter; in the management of papillary or follicular thyroid cancer (alone or with T4); in the treatment of skin disorders such as psoriasis, glaucoma, cardiovascular disease such as in the prevention or treatment of atherosclerosis, and congestive heart failure.
The compounds of the invention can be administered orally or parenterally such as subcutaneously or intravenously, as well as by nasal application, rectally or sublingually to various mammalian species known to be subject to such maladies, e.g., humans, cats, dogs and the like in an effective amount within the dosage range of about 0.1 to about 100 mg/kg, preferably about 0.2 to about 50 mg/kg and more preferably about 0.5 to about 25 mg/kg (or from about 1 to about 2500 mg, preferably from about 5 to about 2000 mg) on a regimen in single or 2 to 4 divided daily doses.
The active substance can be utilized in a composition such as tablet, capsule, solution or suspension or in other type carrier materials such as transdermal devices, iontophoretic devices, rectal suppositories, inhalant devices and the like. The composition or carrier will contain about 5 to about 500 mg per unit of dosage of a compound-of formula I. They may be compounded in conventional matter with a physiologically acceptable vehicle or carrier, excipient, binder, preservative, stabilizer, flavor, etc., as called for by accepted pharmaceutical practice.